My players achieved an Overwhelming success
A size five PC vessel will face two size four shuttles. Shuttles are great while their power holds out, but only having one engine they
really suffer after their initial burst of actions.
I had missile-armed shuttles (swapped in for the science bay - so no shields) and they managed half a dozen launches and beam attacks before being left mostly adrift while they attempted to recover a decent level of power. In the meantime the players eliminated most of the crew in one, and badly damaged the other. In return the players took heavy damage (two engines slagged and one damaged with life support out and the engineer unconscious at one point! - the Silicoid scientist had to
run 
),
The size five vessel will boost only when the first two are out of the fight (either destroyed, or in your opinion no longer effective) and as noted in the scenario it boosts away from the players, attacking to destroy the liner in an attempt to eliminate it, but mostly aiming to hyper out with Free Lunch.
So the size five isn't really combatant as far as the players are concerned (unless they're being extremely effective in stopping it from hypering out). I really only made it bigger so that it was unlikely to be destroyed - that would kind of ruin the follow-up scenario...
Overwhelming Success then only requires that the liner is intact at the end of the scenario (it is the "Tendril" that is a required objective, Free Lunch is no longer part of the victory conditions. It is intended that Free Lunch is dragged off by the pirates),
The way it went it was a tricky scenario, but eminently completable,
